Serpfox checks down to the 300th search result. If your URL was not found in the top 300 results a blank field will be shown in the rank column.
Prior to July, 2017 we tracked down to the 500th search result. At that time only 0.24% of our tracked keywords showed a result lower than 300, so we deemed that it was unnecessary to track lower than 300. If you have a different opinion let us know!